Preparation of unsaturated carboxylic acids by oxidation of unsaturated aldehydes with nickel peroxide in the presence of alkali hydroxide



United States Patent 3,361,805 PREPARATZGN 0F UNSATURATED CARBOXYLIC ACTBS GXIDA'IION 0F UNSATURATED AL- DEEIYDES WlTH NICKEL PEROXlDE IV THE PRESENCE (3F ALKALI HYDRQXIDE Fernholz, Earl ioden, Taunus, and Giinter Jacobsen, Frankfurt am Main, Germany, assignors to Far-bwerke Hoeehst Aktiengeseilschaft vormals Meister Lucius it Brunhig, Frankfurt am Main, Germany, a corporation of Germany No Drawing. Filed Feb. 4, 1964, Ser. No. 342,521 Claims priority, application Germany, Feb. 6, 1963, F 38,937 6 Claims. (Cl. 260-530) ABSTRACT OF THE DISCLGSURE Preparation of monoand polyunsaturated aliphatic and cycloaliphatic carboxylic acids of 4 to 20 carbon atoms by oxidation of the corresponding aldehyde with nickel peroxide in the presence of alkali hydroxide while the reaction mixture is suspended in water.

The present invention relates to a process for preparing unsaturated carboxylic acids.

It is known to prepare carboxylic acids by oxidation of the corresponding aldehydes. A great variety of oxidizing agents are used for this purpose, the kind of the oxidizing agent used depending on the nature of the aldehyde. Gaseous oxygen, for example, is used when the oxidation is carried out in the presence of a catalyst Which serves to introduce the oxygen into the aldehyde. In other cases stoichiometrical amounts of substances that readily yield oxygen, for example, hydrogen peroxide, peroxy acids or potassium permanganate, or higher metal oxides that readily decompose while yielding oxygen, for example, manganese dioxide, lead dioxide, nickel peroxide, mercury oxide or silver oxide, are used. When metal oxides are used the oxidation is preferably carried out in an alkaline solution and the carboxylic acid is obtained after the metal oxide has been separated by filtration and the remaining solution has been acidified,

Under the conditions described above, saturated aliphatic and aromatic aldehydes can in general be converted without any diiiiculty into the corresponding carboxylic acids which are obtained in high yields. This does not apply to unsaturated aldehydes. The double bonds present in the molecule are places which may likewise be attacked by the oxidizing agent and this leads to the formation of more or less large amounts of by-products. In many cases the unsaturated carboxylic acid cannot at all be obtained in a quantity that is worth being mentioned.

Now we have found that unsaturated aldehydes, in particular sorbic aldehyde, can be converted with good yields into the corresponding unsaturated carboxylic acids or into the alkali salts thereof when nickel peroxide is used as an oxidizing agent and the process is carried out in an aqueous alkaline emulsion. The ratio of water to unsaturated aldehyde is generally within the range of 1:1 to 12:1 parts by Weight and advantageously within the range of 5:1 to 7:1. in a preferred embodiment of the invention the aldehyde that has been emulsified in water is first placed into the reaction vessel and the oxidizing agent is then introduced together with the required amount of alkali hydroxide, advantageously in the form of an aqueous solution of 10 to 50%, advantageously to strength. The process is carried out at a temperature Within the range of 10 C. to 80 C., preferably C. to 50 C. The nickel peroxide is advantageously added in small portions, each portion being added only after the colour has changed from black to grey-green, which shows that the preceding portion of oxidizing agent is consumed. When the reaction is terminated, the solution of the alkali salt of the unsaturated carboxylic acid is separated from the nickel hydroxide. When the filtrate is acidified, the unsaturated carboxylic acid can be obtained by filtration in cases in which it is sparingly soluble or by extraction with a solvent, for example, ether.

The nickel peroxide required for the oxidation can be prepared in known manner, for example, by treating an aqueous suspension of nickel hydroxide which may have formed by the precipitation of a nickel sulfate solution with sodium hydroxide solution, with a solution of sodium hypochlorite (soda bleaching liquor). The resulting suspension of black nickel peroxide can be directly subjected to its further rocessing, It is advantageous, however, to remove the solid substances by filtration from the solution which still contains an excess of bleaching liquor and to suspend the solid substances again in water. To the resulting suspension the solution of the alkali hydroxide, e.g., the hydroxide of Li, Na, K, Rb or Cs, preferably potassium hydroxide solution or sodium hydroxide solution, is added. It is suificient to use 1 mol of alkali hydroxide per mol of aldehyde. Advantageously the alkali hydroxide is added in a slight excess, for example, in an amount of 1.2 mols per mol of aldehyde.

The speed of reaction depends on the temperature used. When the temperature of the reaction is within the range of 30 C. to 40 C. the reaction is terminated in general in a period Within the range of 1 to 3 hours. The nickel hydroxide, which is advantageously removed by suction, can again be converted by means of bleaching liquor into nickel peroxide.

Upon acidification of the alkaline solution that has been freed from nickel hydroxide, unsaturated carboxylic acid having a high degree of purity is obtained in a good yield. The aldehyde used can be completely converted.

The process according to the present invention can be applied to monoor polyunsaturated aliphatic or cycloaliphatic aldehydes containing 4 to 20 carbon atoms, for example, crotonic aldehyde, sorbic aldehyde, octatrienal and tetrahydrobenzaldehyde.

It is a particular advantage of the process according to the invention that it enables even very sensitive aldehydes to be converted into the corresponding carboxylic acid under mild conditions of oxidation, It is substantially only the aldehyde group that undergoes a change while the rest of the molecule is not attacked.

The unsaturated carboxylic acids which are prepared by the process according to the invention can be used for many purposes. Sorbic acid, for example, is important as a preservative for food-studs.

The following examples serve to illustrate the invention but they are not intended to limit it thereto.

Example 1 48 grams of sorbic aldehyde (0.5 mol) were emulsified in 300 cc. of water while stirring vigorously.

When the mixture had a temperature Within the range of 30 C. and 35 C. a suspension of nickel peroxide in 400 cc. of Water which had been combined with a solution of 22 grams of sodium hydroxide (0.55 mol) in 87 cc. of water was added in small portions to the emulsified aldehyde, every portion being added after the colour of the nickel compound from the preceding portion had turned from black to greyish-green. The reaction was terminated within a period of 2 to 3 hours.

The nickel peroxide was prepared by dissolving 421 grams of NiSO .7H O (1.5 mols) in 500 cc. of water, adding to the resulting solution drop by drop at room temperature, while stirring, a mixture of grams of sodium hydroxide (3.75 mols) dissolved in 600 cc. of water, and 550 grams of soda bleaching liquor of 12.7% strength (0.95 mol), whereupon stirring was continued for half an hour. The black precipitate was filtered off, washed with water and suspended in water in the manner described above Without drying before.

After the oxidation was terminated the alkaline solution was isolated from the nickel hydroxide by suctionfiitration and washed with water. The slightly yellow filtrate was then acidified to a small extent with dilute mineral acid. The precipitated sorbic acid was filtered off, washed with Water and dried in vacuo over calcium chloride.

38.8 grams of an almost pure sorbic acid were obtained which corresponds to 69%, calculated on the sor'oic aldehyde used, of the theoretical yield.

When using instead of sodium hydroxide the equivaent' amount of potassium hydroxide the same favourable results were obtained.

Example 2 35 grams of crotonic aldehyde (0.5 mol) were oxidized with nickel peroxide in the manner described in Example 1.

Due to its good solubility in water the crotonic acid which had been set free by removing the nickel hydroxide by suction-filtration and acidifying the alkaline filtrate had to be separated by extraction with an appropriate solvent which in this case was ether. After the solvent had been removed by distillation, 36.6 grams of pure crotonic acid remained behind which corresponds to 85% of the theoretical yield.

We claim:

1. A process for preparing a monoor poly-unsaturated aliphatic or cycloaliphatic carboxylic acid of 4 to 20 carbon atoms by oxidation of the corresponding aldehyde which comprises suspending said aldehyde in from one to twelve parts by weight of water per part by weight of aldehyde, reacting the suspended aldehyde with nickel peroxide at a temperature within the range of 10 to 80 C. in the presence of l to 1.2 mols alkali hydroxide per mol of aldehyde, separating the nickel hydroxide thus formed, and acidifying the reaction mixture after termination of the reaction.

2. A process as claimed in claim 1 wherein the reaction is carried out at a temperature Within the range of to C.

3. A process as claimed in claim 1 wherein sorbic aldehyde is oxidized.

4. A process as claimed in claim 1 wherein crotonic aldehyde is oxidized.

5. A process as defined in claim 1 wherein the reaction of the aldehyde with nickel peroxide is carried out by adding small increments of nickel peroxide and alkali hydroxide suspended in water to the aqueous suspension of aldehyde.

6. A process as defined in claim 1 wherein the initial aldehyde is suspended in from 5 to 7 parts by weight water per part by weight of aldehyde.

References Cited UNITED STATES PATENTS 2,744,929 5/1956 Smith et a1. 260-530 LORRAINE A. WEINBERGER, Primary Examiner.

V. GARNER, Assistant Examiner. 

